What Are the Differences Between Twitter SMS?

Interface
Although desktop software programs like Skype and Yahoo! Messenger are capable of sending SMS text messages to phones, you almost exclusively use a cellphone to send and receive text messages. On the other hand, it's possible to use Twitter not only from your laptop or desktop computer, but also on an Internet-ready mobile phone. If your phone doesn't have Internet, you can update your Twitter -- and view others' updates -- via SMS text message, an overlap between the two technologies.
Scope and Influence
One of Twitter's key features is the concept of 'Following' -- each user 'follows' other users and a certain number of other users, which may or may not be among those he or she follows, follow that user. As a result, each message a Twitter user sends out has the potential to reach hundreds or even thousands of users, depending on how many users follow him or her. SMS text messages, on the other hand, are generally sent from person-to-person.
Purpose
Twitter and text messaging also differ in their intended purpose. While people send SMS text messages to one another mostly to privately communicate specific messages and feelings, Twitter has a wide range of applications. These include posting multimedia, advertising businesses and generating publicity. It's possible to send direct, private messages on Twitter, although this tool is arguably secondary to the main Twitter feed -- and the public tweets users post to it.
Cost
Although many cellphone plans these days include the option to add unlimited text messages for a fixed monthly fee, text messaging is, at its root, a service which incurs cost. Twitter, on the other hand, is completely free to use. If you do access Twitter from a cellphone that doesn't have Internet access, the tweets you send to Twitter via text may incur a charge or count against your monthly allotment if you haven't signed up for an unlimited plan.